In addition, copycat versions of the virus which use a different subject line, message body, and/or Word document attachment, but the same essential virus code have been seen, though none of these spread nearly as far and wide as the original.

When it first infects your computer, if you have any of the variants of Microsoft Outlook installed, it reads the first 50 addresses from the Address Book (or all of them, if there are less than 50) and mails itself to those people. In this sense, it is similar to the Internet Worm.

Consequences
At a lot of companies, including my own, the default Address Book is a company-wide shared address book, and in some cases there is an "all employees" mailing list among the first 50 addresses, so it can easily replicate across an entire company, even one with far more than 50 employees.
Those occasional users who use their own address book as a primary address list then allow it to escape the company and go on to their friends elsewhere.

In some cases, the sheer volume of e-mails crashed mail servers, and in many cases the high mail load alerted admins quickly that something was going wrong.